🐛 Resolves asciidoctor#240, set the base directory

Otherwise Firefox cannot resolve include directive with relative path.

app/js/renderer.js

@@ -277,8 +277,12 @@ asciidoctor.browser.renderer = (webExtension, document, Constants, Settings, Dom
     if (attributesQueryParameters.length > 0) {
       Array.prototype.push.apply(attributes, attributesQueryParameters);
     }
+    const parts = href.split('/'); // break the string into an array
+    parts.pop(); // remove its last element
+    const pwd = parts.join('/');
     return {
       'safe': safeMode,
+      'base_dir': pwd,
       // Force backend to html5
       'backend': 'html5',
       // Pass attributes as String